Orders: Copepoda, Cladocera, Siphonostoma, Lernaeida, Cirripedia.
Orders: Branchiopoda (Sub-orders: Phyllopoda, Cladocera, Branchiura), Ostracoda, Copepoda.
Sars (1887) having had the opportunity of raising it from dried Australian mud, found that, unlike other phyllopods, but like the Cladocera, the parent keeps its brood within the shell until their full development.
The great majority of the Cladocera belong to fresh water, but their adaptability is large, since Moina rectirostris (0.
For Streblocerus pygmaeus, grown under the same conditions, Sars observes: " This is perhaps the smallest of the Cladocera known, and is hardly more than visible to the naked eye," the adult female scarcely exceeding 0.25 mm.
Certain waterfleas (Cladocera) fall short of one-hundredth of an inch in total length; the giant Japanese crab (Macrocheira) can span over Io ft.
In many Entomostraca (Phyllopoda, Cladocera, Ostracoda, Copepoda) they are important, and sometimes the only, organs of locomotion.
A progressive abbreviation of the heart, with a diminution in the number of the ostia, can be traced, leading to the condition found in the closely related Cladocera, where the heart is a subglobular sac, with only a single pair of ostia.
The eggs are retained between the valves of the shell in some Phyllopoda and in the Cladocera and Ostracoda, and they lie in the mantle cavity in the Cirripedia.
The Phyllopoda must have branched off very early and from them to the Cladocera the way is clear.
A few cases are known in which the developing embryos are nourished by a special secretion while in the brood-chamber of the mother (Cladocera, terrestrial Isopoda).